This invention relates to digital electronic controllers and, more particularly, to portable digital electronic load cell indicators.
Measuring transducers are known in the art and are used to measure forces associated with any number of applications involving such things as weight, gravity, pressure, and vibration. A load cell is a common measuring transducer that is typically used in a wide variety of scales or weighing systems. A load cell typically generates an analog signal that is converted to a digital signal before being displayed or otherwise processed or used. An electronic controller, such as a load cell indicator, is typically used to display data received from a load cell and is often used to convert the data from analog to digital format before the data is displayed or used.
Load cell indicators are well known. Most have common weaknesses that present problems particularly when the load cell indicators are used in rough, washdown environments. In some environments, such as poultry processing facilities, the indicators may be handled roughly and with disdain. Cleaning crews may aim high pressure spray guns spraying hot cleaning solution at electronic instruments for sport. This high pressure spray will often seep past gaskets and seals, leaving the internal electrical connectors and electronic components wet. Since the cleaning solutions used are typically slightly conductive, any solution that finds its way to the incoming AC power connector will often result in damage, which can range from minor corrosion of wiring and printed circuit board traces to arc-over and destruction of the AC power connector and related circuitry. This damage often limits the useful life of an instrument to mere weeks.
The personnel using the indicator as part of their daily job function may also be handling sharp pointed instruments, such as knives or scissors. They quite often use the sharp instrument instead of a finger to press buttons on a keypad. If the indicator uses a plastic membrane keypad, which is by far the most common type of front panel found on load cell indicators, this activity will quickly result in a damaged keypad. Depending on the design on the specific indicator, a damaged keypad may be replaced, or it may mean replacement of the entire indicator. Again, this potential for damage limits the lifetime of indicators in some environments.
To prevent damage from the user or the environment, some load cell indicators are mounted inside another enclosure, often with a latching flip-up transparent front lid, allowing the user to see the display. To access any of the front panel controls, the user must first unlatch the lid, lift it up, and finally press the desired key. While this style of double enclosure mounting may reduce high pressure fluid infiltration in the indicator enclosure, it also limits operator access to the indicator, sometimes excessively so. The effectiveness of this mounting style is frequently reduced by the tendency of operators to forget to latch the front lid, leaving the indicator exposed to the very high pressure spray the lid was intended to prevent.
Most digital load cell indicators have some type of display to present weight information to the user. The choice of display can be problematic, as many display types with outstanding visibility (such as a vacuum florescent display—VFD) have high power consumption. Indicators that use VFDs often have no provision for running from battery power, limiting their usefulness as a portable device. Other indicators use seven-segment light emitting diode (LED) displays, which provide good visibility and reasonable power consumption, but the seven-segment style of digit is not conducive to presenting alphanumeric data to the user. Indicators with seven segment displays are thus limited in displaying information, which makes setup and troubleshooting cryptic and difficult. LCD (liquid crystal display) technology is also used in load cell indicators. LCDs are very low power consumption devices. LCDs are also available in character mode and dot-matrix configurations, which allow the presentation of alphanumeric data and even graphic information in an easily readable style. However, many digital load cell indicators use LCD modules that require ambient light for the display to be visible to the user, which makes the instrument difficult to use in dark or uncertain lighting.
Load cell indicators designed for use in washdown environments are often housed in stainless steel enclosures to resist corrosion. High temperature washdown followed by extended use in cold temperature environments can lead to condensation forming on the enclosure. If the seals around the cables, display, and keypad are less than perfect, and they usually are, condensation can also form on the inside of the enclosure, which can wet the electronic components, circuit boards, and connectors. Display fogging is also common in this situation. These conditions conspire to limit the effectiveness of the indicator.
Load cell indicator enclosures designed for washdown environments are sometimes designed with many closely-spaced fasteners and gaskets to prevent or reduce high pressure fluid infiltration. If internal access to the enclosure is desired, such as to replace a load cell cable or power cable, the technician performing the procedure will spend quite a bit of time and frustration removing all the fasteners and sealing material to gain access to the internal connections of the indicator.
Very few load cell indicators are designed to interface with more than one independent load cell. This limitation leads to the use of external hardware, such as summing boxes, to condition multiple load cell signals (as might be provided by a large tank or platform scale) for use with common indicators. Summing boxes are generally passive devices, with potentiometers that must be manually adjusted to ensure that each load cell contributes equally to the summed analog signal. It is not uncommon for a scale technician to spend hours manually adjusting a single load cell summing box to allow an indicator to accurately weigh when connected to a multiple load cell platform.
Some load cell indicators have provisions for interfacing with other instruments, such as bar code label printers, data collection devices, or computer hardware. This connection is often through a serial communications port. Depending on the requirement of the user, the data transfer format between the load cell indicator and other auxiliary instruments, may have to be customized to the user's specific installation and circumstances. For many load cell indicators, this is simply not possible, as there is no allowance for field customization of the software code contained inside the indicator controller, and the manufacturers are unable to update software once the indicator has been manufactured.
Advanced or high-end load cell indicators may be used as weighing machine controllers, in which case they are often integrated into other, more complicated machines. These machines might consist of several load cells, other load cell indicators, various valves, slides, gates, and motor drives, sometimes from a diverse group of manufacturers. During integration and testing, problems, bugs, or unforeseen circumstances will often arise that require the machine controller software to be modified. If the advanced load cell indicator/controller is not designed with this in mind, the upgrade might require significant disassembly of the indicator and associated hardware to replace a physical electronic device containing the controller software.
Many users of load cell indicators desire a small, portable instrument that can be mounted on a load cell platform and that can be carried by hand. This requirement limits the overall size and weight of the instrument and also requires a portable power source, such as a battery. Many load cell indicators do not have any provision for a battery. Some do, but require so much power that the only practical option is a physically large battery mounted external to the indicator enclosure. Any externally mounted battery will require wiring to provide power to the indicator, and that wiring introduces at least two additional weaknesses into the system: the wiring must be sealed to prevent leakage where it enters the indicator enclosure, and exposed wiring is vulnerable to damage.
Load cell indicators used in harsh environments are often damaged. When this occurs, many users will desire to troubleshoot the indicator to determine the cause of the failure, and then will often elect to attempt a repair. Load cell indicators are relatively complicated and include delicate electronic instruments, so troubleshooting may call for the use of skilled electronic technicians and other highly trained and costly personnel equipped with an assortment of specialized tools. If the indicator does not have adequate documentation or if spare parts are not easily available, it may be more economical to replace the entire indicator rather than to repair it. A user or technician who may attempt troubleshooting but who may lack the proper training or the right tools can often do more harm than good when provided with ready access to all of the electronic components of these relatively complex, delicate instruments.
All load cell indicators will require calibration to accurately weigh when interfaced with a specific load cell. Indicators are also calibrated at regular periods of time to meet various legal, trade, and internal requirements. Many load cell indicators are difficult to calibrate. The indicator calibration procedure may require various internal switches and potentiometers to be manually adjusted in a very fine and precise manner. The calibration procedure may require an exact value of weight to be applied to the scale. Some indicators have a rather lengthy and complicated calibration procedure that is not obvious without supplemental documentation, so if the calibration procedure instructions are misplaced, that scale is unusable.